Hemophilia B is a bleeding diathesis due to a deficiency of blood coagulation Factor IX (F.IX). AAV-mediated,[unreadable] liver-directed gene transfer has yielded long-term (>5 years) expression of therapeutic levels of[unreadable] F.IX in the canine model of the disease. A clinical study based on these[unreadable] findings uncovered obstacles that had not been evident in pre-clinical studies. The first subject treated at a[unreadable] therapeutic dose initially demonstrated F.IX levels of approximately 10-12% for 4 weeks, but then F.IX levels gradually[unreadable] returned to the baseline level of <1%. The decline in F.IX levels was accompanied by a mild, self-limited[unreadable] transaminitis, that began 4 weeks after vector infusion and fully resolved several weeks later. Transient[unreadable] transaminitis was observed in a second subject and immunologic studies in this subject documented a T cell[unreadable] response to AAV capsid. The goal of this application is to understand, in terms of the cellular and humoral[unreadable] immune response, what happened to these subjects, and whether more detailed characterization of immune[unreadable] responses to AAV capsid will permit us to identify subjects likely to benefit from AAV-mediate, liver-directed[unreadable] gene transfer. In addition, we prepare to determine whether immunomodulatory therapies, or[unreadable] changes in the vector, can alter the outcome in favor of prolonged expression. To accomplish these goals, we[unreadable] shall pursue studies in murine and non-human primate animal models, and in normal and hemophilic subjects.[unreadable] In the first aim, we will examine T cell responses to AAV-2 capsid sequences in the normal human[unreadable] population, in hemophilic patients, and in hemophilic subjects who have been injected parenterally with AAV[unreadable] vectors. The second aim will determine how long AAV capsid proteins persist in an immunologically[unreadable] detectable form following introduction of vector into the livers of mice. In the third aim, we shall examine T[unreadable] cell responses to AAV-8 and to A modified Rhesus F.IX in non-human primates injected with an AAV-8[unreadable] vector expressing F.IX. AAV-8 is a simian AAV; because NHP may be naturally infected prior to vector[unreadable] infusion, this may more accurately model the series of responses in humans infused with AAV-2 vectors.[unreadable] Lymphocytes from both liver and peripheral blood will be examined in these studies. These studies will be[unreadable] critical for developing an understanding of pre-existing immunity to AAV and its implications for AAV-mediated[unreadable] gene transfer.